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I live on a hill where the soil has lots of clay. My back garden has been terraced with small (1m) retaining walls. The net result is that we have several lawns / gardens with the following properties:

  1. Surrounded on all sides by concrete retaining walls.
  2. The top 20-30cm is good soil; the remainder is clay.
  3. At the bottom of some of the retaining walls (i.e. a bit less than 1m below the surface), there are periodic drainage holes.

So the drainage is quite terrible -- the water can't get out the sides, and it can only sink slowly through the clay. To improve this, my idea was to dig a trench ~30cm wide along the side of the retaining walls (where the drainage holes are), fill the bottom 50cm with sand, and the rest with dirt, and then run drainage pipes from the rest of the lawn to this trench.

On the other hand, I have also read: "Don't mix sand and clay; that's how you get brick". Is this a good idea? Will it work? Are there better options?

Edit: Photos below (excuse the laundry -- we can't let a sunny day go to waste :-) )

The corner with the manuka (centre first image, bottom-right second image) is evidently a little lower than the rest of that lawn, and thus in heavy rain it can be under 5+ cm of water.

Southern lawn Northern lawn

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Can you please post one or two photos showing the hardscape/landscape you're dealing with. Take a look at this answer here on SE Gardening, especially take note of the "French Drain" information given (as I believe that would probably offer you the best solution based on the very little I know of your hardscape/landscape). I most definitely wouldn't back fill with sand -- a much better solution would be a combination of landscape fabric & pea-gravel (IMHO). –  Mike Perry Oct 30 '11 at 0:34
    
Before I write up my answer (suggestion), can you please explain "... and then run drainage pipes from the rest of the lawn to this trench." as I don't currently fully understand what you mean by it (I'm unable to picture it in my mind)... –  Mike Perry Oct 30 '11 at 6:20
1  
In my mind, it works like this: 1. If I improve drainage at the retaining walls, water will have a way to get out. 2. Water elsewhere on the lawn needs a way to get to the walls. ==> 3. Underground pipes! (I guess I should call them french drains?) –  John Fouhy Oct 31 '11 at 6:37
    
Additionally you could look to improve the (clay) soil overtime eg How do I work with soil that has a high clay content? & Improving extremely clayey soils –  Mike Perry Oct 31 '11 at 19:47

4 Answers 4

up vote 7 down vote accepted

Below is what I would consider a "Rolls-Royce" job, all of it might not need to be done if water doesn't puddle on the surface of the lawn...

French drains on the back face/side of the upper and lower retaining walls:

  • Dig 300 to 450mm (12 to 18inches) wide trenches.

    • The trenches should run the length of the retaining walls.

    • The trenches should bottom out 100mm (4inch) below the existing drainage holes in the retaining walls.

    • Keep the "good" top soil for back-fill material.

  • Clean out the existing drainage holes in the retaining walls.

  • It's a little difficult to tell from the photos, but if you feel there are not enough drainage holes in the retaining walls or they are spaced greater than 2000mm (78inches) centers, I would recommend drilling some additional drainage holes through the retaining walls.

    • Layout any new drainage holes at the same level as the existing ones and position them along the retaining walls as needed -- no greater than 2000mm (79inches) between them.

    • First drill pilot holes through the retaining walls using a 12.5mm (½inch) masonry drill bit, then use a 50mm (2inch) concrete core cutter to drill the "final size" drainage holes.

  • Get some outdoor screen mesh fabric (small off-cuts) and place the pieces on the back face/side of the retaining walls over the drainage holes -- this will help keep them clear of debris and functional.

  • Get enough "permeable" landscape fabric to line the trenches.

    • The landscape fabric should go 200mm (8inch) up the back face/side of the retaining walls, lay across the bottom of the trenches, then up the face of the trenches (opposite the retaining walls) to above ground level.
  • Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

  • Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

    • The drainage pipes should run the length of the trenches (retaining walls).
  • Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

  • Fold over the landscape fabric so it covers the top of the pea-gravel.

  • Finish back-filling the trenches with the saved top soil back-fill material.

  • Make good the surface to match in with the existing.

French drains on the outer face/side of the upper retaining walls:

  • Dig 300 to 450mm (12 to 18inches) wide trenches.

    • The trenches should run the length of the upper retaining walls (minus the path/walk-way in the middle).

    • The trenches should bottom out at the same level as the trenches dug behind the lower retaining walls.

    • Keep the "good" top soil for back-fill material.

  • Get enough "permeable" landscape fabric to line the trenches.

    • The landscape fabric should go 200mm (8inch) up the outer face/side of the upper retaining walls, lay across the bottom of the trenches, then up the face of the trenches (opposite the retaining walls) to above ground level.
  • Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

  • Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

    • The drainage pipes should run the length of the trenches (upper retaining walls, minus the path/walk-way in the middle).
  • Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

  • Fold over the landscape fabric so it covers the top of the pea-gravel.

  • Finish back-filling the trenches with the saved top soil back-fill material.

French drains on the lower level lawn areas, running perpendicular between the upper and lower retaining walls:

  • Dig 300 to 450mm (12 to 18inches) wide trenches.

    • 3 number per lower level lawn area.

    • 1 on each of the inside faces of the "side/wing" retaining walls. Position 1000mm (39inches) away from the "side/wing" retaining walls.

    • And the 3rd positioned in the middle of the 2 (near the "side/wing" retaining walls).

    • The trenches should run between the trenches on the outer face/side of the upper retaining walls and the trenches on the back face/side of the lower retaining walls.

    • The trenches should bottom out at the same level as those trenches.

    • Keep the "good" top soil for back-fill material.

  • Get enough "permeable" landscape fabric to line the trenches.

    • The landscape fabric should go up one face of the trenches to above ground level, lay across the bottom of the trenches, then up the opposite face of the trenches to above ground level.
  • Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

  • Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

    • The drainage pipes should "T" into the drainage pipes that are running along the "face" of the upper and lower retaining walls
  • Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

  • Fold over the landscape fabric so it covers the top of the pea-gravel.

  • Finish back-filling the trenches with the saved top soil back-fill material.


If water doesn't puddle on the surface of the lawn I would considering not doing the work under heading "French drains on the lower level lawn areas, running perpendicular between the upper and lower retaining walls."


Good luck! and I hope the above makes some kind of sense (is somewhat helpful/useful). If you have any questions please ask, I will then do my best to address them...

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Very interesting -- thanks! I plan to start (and hopefully finish) this over Christmas / New Year. Any tips for trench digging? The top one will be quite deep -- I've never dug that deep before. –  John Fouhy Nov 1 '11 at 7:34
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@JohnFouhy "Generally" unless the soil is "loose" you don't need to worry about shoring up the sides of trenches (holes) 2m (6½ft) deep or shallower, though Health & Safety (in the UK) says differently... From the photos I'm guessing the trench behind the upper retaining wall will be about 1.2 (4ft) deep, therefore there should be no need for shoring of the sides, but obviously soil conditions will ultimately dictate if shoring is required or not... –  Mike Perry Nov 1 '11 at 14:59
    
@JohnFouhy If you need to dig down deeper than 900mm (3ft), you will probably find it much easier to widen the trench ie Make the trench 500 to 600mm (20 to 24inches) wide. –  Mike Perry Nov 1 '11 at 15:03

Plant daikon radishes in the area affected these radishes are 3 feet deep/long. After each plant makes these "holes" and rot. You will have better drainage. The rotting root/radishes will become humus which will also make the soil more drain-able. effect Seeds are cheap and the leaves and root are also edible.

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Depending on the level of work you are willing to put into it...

You could use someone like Haydite (http://www.haydite.com/)or another expanded shale product. It's cheap they make it all over the country so it's not to difficult to come by.

Then you dig up your terraces mix the Haydite to the right consisnancy (you can ask the source for instructions, especially if you bring a sample of what your working with) and re-seed your lawn.

It's fairly cheap and straight forward. Just some excavation wich is not so small.

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I agree with the very helpful advice above, but depending on the thickness of the clay, you may find a soakaway trench doesn't work (the trench would just fill, like a pond). You may need to drain the water into the the drainage system, fairly easy if the slopes work, If not build a sump and pump the water into the drainage system.

Hope this helps you can find information for garden drainage and sump pumps here. http://www.flowerpotman.com/lawnsandgardendrainage.html

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